In the use of vehicles in extremely cold environments, as for example a crawler tractor in Alaska during the winter, an operator sometimes finds that the hydraulic fluid increases in viscosity to a magnitude sufficient to make the hydraulic system slow to respond. One of the principal locations at which the hydraulic fluid becomes excessively viscous is in the relatively long conduits which provide the fluid pathway between the control valve and the hydraulic cylinders which actuate the clutch and brakes of the crawler tractor. During extended operation without clutch actuation, the fluid in these long conduits can cool to a value at which the pump pressure is slow to move the viscous fluid in the conduit upon actuation of the control valve.
This invention solves the problem of hydraulic line "freeze up" during inactive periods of the associated work element by providing means for substantially continuously passing hydraulic fluid between the control unit and the hydraulic cylinder.